1. Field of the Invention
The present invention relates to a mobile object image tracking apparatus and method for enabling a target recognition sensor, such as a camera, to track a target that can move in every direction.
2. Description of the Related Art
In recent years, systems for tracking an object using, for example, an ITV camera, to realize continued monitoring or acquire detailed information have been produced on a commercial basis for protective service equipment employed in major facilities such as airports and manufacturing plants, lifeline facilities such as electric power plants and water networks, and traffic information support systems such as Intelligent Transport Systems (ITS). These systems include not only ground equipment type systems, but also compact ones installed in vehicles, ships or airplanes and having a vibration-proof structure. In the systems, it has come to be important to enhance their whirling speed to make them quickly point targets and sequentially track the targets.
In the above mobile object image tracking systems, their object tracking velocity is limited by the field of view of a camera sensor employed. Namely, unless a tracking error that occurs when the camera sensor photographs a mobile object falls within the field of view, the mobile object will be missed. Such a tracking error occurs because the camera sensor performs image processing for extracting a tracking error from an image obtained by photography, and hence it is difficult to reduce the interval of photography, i.e., the sampling time, of the camera sensor. This being so, when the velocity of a mobile object is increased, the object may fall outside the field of view of the camera sensor and become unable to be tracked. Further, depending upon the attitude of a gimbal mechanism employed in the systems, it is possible that the gimbal mechanism itself will instantly move at high velocity to avoid gimbal lock. In this case, the mobile object may well fall outside the field of view of the camera sensor as in the case where the velocity of the mobile object is increased. Once thus the mobile object falls outside the field of view, it is difficult to return the same from outside the field of view to inside. In addition, if the sampling interval of the camera sensor is long, an angular velocity instruction for causing the gimbal mechanism to perform tracking also is updated at long intervals. Namely, the sampling time of the control system becomes long, and this adversely affects the tracking performance, with the result that a mobile object falls outside the field of view of the camera sensor because of a tracking delay.
There is a conventional image tracking system, in which when a mobile object has shifted from inside the field of view of a camera sensor to outside, an angular velocity instruction is switched from an instruction based on a tracking error of the camera sensor, to a preset instruction, thereby pointing a gimbal mechanism toward a position outside the field of view so as to return the mobile object into the field of view (see, for example, JP-A 2006-106910 (KOKAI)).
Further, there is a conventional method, for use in an image tracking system, of overcoming a tracking disabled state using estimation. For instance, there is a method of estimating and correcting the position of a target when image information of the target cannot be acquired, by measuring the distance between the target and the image tracking system using a length-measurement sensor.
Furthermore, there is a method of compensating for tracking control by changing a tracking mode, based on the course of a target determined from the size of the target and a change in the center of gravity of the target that are detected in photographed images of the target.
In these techniques, it is difficult to issue an angular velocity instruction that enables a mobile object to be always appropriately returned into the field of view of the camera sensor. Namely, only limited occasions can be dealt with. Further, the method of measuring the distance between a target and an image tracking system requires a length-measurement sensor, and costs high. In the method of compensating for tracking control based on the size of a target and a change in the center of gravity of the target, load on image processing is increased and hence the sampling time is increased. Therefore, improvements for the above-mentioned tracking delay due to an increase in sampling time cannot be expected.